Objects are not selectable since Blender 2.8 UI Freeze
In case anyone else comes across this, the selectability toggle is now filtered by default. What I mean by that is in the top right corner of the Outliner you'll see a filter icon:


Opening it will provide you with the various toggles and options for objects in the outliner. There you can enable the selectability toggle:



That should be all, hope this helps whoever comes by!